One of the most expensive items on a building is the roof structure. Generally for permanent structures beam and rafter frameworks are used which require skilled labour both in their manufacture and in the erection of the roof structure. They are also an expensive component of the building particularly when related to the cost of a sub-economical dwelling for example. Frequently roof structures are over designed and the cost of materials used are out of proportion to the basic function and requirements of a roof. Also roof covering sheets are invariably perforated by fixing devices such as screws, bolts and nails thereby introducing a potential leak factor.
Further the fixing of ceilings and insulation under roofs is problematic requiring expertise and materials for effective fixing.
The roofing structures usually have structural members which are provided extensively for these structures and cannot be used otherwise in the complete building.